What should be true about -5 and 5? Why?

Step 3 Reason: If in a triangle two sides are equal, then it is an Isosceles Triangle.

Step 4 Reason: The Sum of interior angles of Triangle is 180°.

Statement of Step 5: Isosceles Triangle Theorem states that ,the two equal sides of Isosceles triangle have equal opposite angles.

Reason for step 9: Addition Subtraction Property

Reason for step 10: ∠ADC and ∠EDA, lie on a straight line,CE.Also, AD is a ray,through point D.

Statement for Step 11: Linear pair Postulate states that,the two angles which are adjacent of a common ray on a line have the sum 180°.

Reason for step 16: Dividing both sides by -2.

Statement for step 18: ∵ m∠CAE=90°

Statement for step 20: CA ⊥ AE

Reason for step 21: If the angle between radius and the point of contact of a line or line segment which  passes through the end of segment of  radius which lies on circle is of 90°, then that line is tangent to the circle.